Controlling the behavior of light transmission and reflection is the basis of many technologies, including such disparate technologies as flat panel displays, smart architectural windows and walls, eyewear, as well as a myriad of military devices.
Liquid crystals (LC) have been used for electro-optic applications and have found commercial success in a number of markets, most notably information displays and privacy window applications. Their operation has fallen into several distinct functional areas, each having its own performance criteria.
Twisted nematic, super twisted nematic, and similar liquid crystal based devices employ polarizers to control light transmission or reflection. In these systems, the change in the transmission of light is accompanied by very little change in the haze or scattering. In other words, the systems are preferentially designed to have low scattering or haze in all states of operation. In fact, the addition of scattering or haze can have a detrimental effect on the system's performance, since it can alter the polarization state parasitically.
Guest-host systems, such as, e.g., U.S. Pat. No. 6,239,778, employ a non-scattering liquid crystal host together with a dichroic dye “guest.” Application of a voltage alters the orientation of the host liquid crystal and guest dye molecules, thereby altering the effective dye absorption cross-section and thus, the transmission of an incident light. Much like polarizer based systems these devices operate with very little change in scattering or haze. As in the polarizer-based systems, scattering is considered a parasitic effect that is avoided.
On the other hand, a different class of devices uses scattering effects that are useful for their operation. These devices use polymer dispersed liquid crystals (PDLC), Polymer Stabilized Cholesteric Texture (PSCT), NCAP, and Dynamic Scattering systems. In these systems, light transmission is controlled by inducing a significant amount of scatter in the material. Although dyes can be added to the system, their function is merely to introduce color or to enhance the scattering effect. These systems all exhibit a significant alteration in the scattering or haze between the two operational modes. Scattering can be caused by liquid crystal droplets such as in PDLC, focal conic texture such as PSCT, or dynamic turbulence induced by electro-hydro-dynamics, etc. However, in these systems, there is some residual haze even in their low-haze states, which is unacceptable for many optical applications.
Many applications cannot be served by either of the above methods alone. In other words, the application would require both a scatter free change in the transmission and/or a scatter based change in transmission. Furthermore, some applications have a specific fail-safe requirement, which is often not easy to implement on most light control technologies.
For example, there is recent interest in see-through near-eye display devices that when worn, enable the user to see his/her surroundings as well as an image displayed on the device near the wearer's eye (e.g. helmet mounted displays or eyewear devices such as Google Glass). In such applications, the quality of the displayed image depends on the amount of ambient light and allowing the user to reduce the amount of ambient light reaching the eye is advantageous when ambient light is bright. Adjusting ambient light allows the brightness of the displayed image to remain the same. In some instances, it is advantageous to completely block any ambient light from reaching the eye, so that the user is “immersed” in the displayed image.
To address this, it is possible, for example, to stack two different liquid crystal devices on top of each other. In this case, the first device would be driven by one drive circuit and would alter its transmission without change in the scattering or haze, and the second device, which would be driven by a second drive circuit, would alter its scattering state with an applied voltage.
Those skilled in the art realize that this would significantly increase the cost and complication of production and the weight of the device. Furthermore, the device would not function as well, since combining the two systems in a tandem method means that the system carries the shortcomings of both technologies at all times.
There is a great need and desire to be able to accomplish these tasks within a single device. In other words, to combine the functionality of transparent low-tint to high-tint liquid crystal devices with the transparent to opaque capability of the second group of liquid crystal devices to achieve a single device that can switch between all of the above-mentioned states.